Selective determination of thiram residues in fruit and vegetables by hydrophilic interaction LC-MS

Thiram belongs to the most important class of dithiocarbamate (DTC) fungicides including dimethyldithiocarbamates (DMDs), ethylenebis(dithiocarbamtes) (EBDs) and propylenebis(dithiocarbamates) (PBDs). During the surface extraction of fruit and vegetables for the LC-MS determination of residues of DMDs, EBDs and PBDs, thiram is reduced by the penicillamine buffer to the DMD anion, thus resulting in false-positive findings of DMD fungicides like ziram. Therefore, an alkaline sulfite buffer was applied for surface extraction, quantitatively transforming thiram into the DMD anion and a stable DMD-sulfite adduct that was used as a selective marker for thiram. Separation was performed isocratically on a ZIC-pHILIC column with acetonitrile-10mM ammonium hydroxide solution (85/15). Mass selective detection was carried out on a single-quadrupole mass spectrometer coupled to an electrospray ionisation interface operating in negative mode. Using d(12)-thiram as the internal standard, recoveries of 80-108% were obtained from apples, tomatoes, grapes and sweet peppers, spiked in the range of 0.02-1mgkg(-1). Limits of detection and quantification were 0.6 and 2 mu gkg(-1), respectively.